The invention relates to a cathode-ray tube comprising in an evacuated envelope an electron gun for generating at least one electron beam which is focused on a display screen to form a spot and which is deflected in two mutually perpendicular directions so that a raster is written on the display screen. The electron gun comprises a cathode which is centered on an axis, a first grid at some distance thereform along the axis and a second grid at some distance from the first grid. The first and second grids each have a part which is perpendicular to the axis and which has an aperture around the axis. The portion of the aperture in the first grid on the side of the second grid is elongate in a direction coinciding with a direction of deflection, and the portion of the aperture in the first grid on the side of the cathode is elongate in a direction perpendicular to the elongate direction of the aperture on the portion of the side of the second grid.
Such a cathode-ray tube may be used for displaying television pictures. It may be, for example, a colour display tube, a monochrome display tube, a display tube for displaying letters, digits and characters (a so-called Data-Graphic-Display tube or D.G.D.-tube) a projection television display tube or an oscilloscope tube. In all of these tubes, particularly at large beam currents, a spot is desired on the display screen which has small dimensions and a minimum of haze around the spot. This is necessary so as to be able to display sharply small details, for example letters, also in the corners of the display screen.
Such a cathode-ray tube is known from the U.S. Pat. Nos. 4,242,613 and 4,358,703 which may be considered to be incorporated herein. It is described in these patents that the haze around the spot on the display screen, also in the corners and at the edge, can be reduced considerably by means of a cathode-ray tube as described in the opening paragraph. By constructing the first grid as described, an astigmatically-focused electron beam is obtained, which is less deformed by the deflection coils, which produce an astigmatic electron lens. In a cathode-ray tube the spot of the electron beam on the display screen is the reproduction by means of one or more electron lenses of a cross-over which is present in the region between the first and the second grid. By constructing the first grid as indicated, not one cross-over is obtained, but the electron beam originating from the cathode is focused in two focal lines present at a distance from each other, and is then focused on the display screen to form a spot.
Another manner of improving the spot quality is to reduce the influence of spherical aberration. This manner is described in Netherlands patent application Ser. No. 8204185 (corresponding to U.S. patent application Ser. No. 544,169 filed Oct. 21, 1983) not yet laid open to public inspection, and which may also be considered to be incorporated herein. In the cathode-ray tube described in the above mentioned patent application, viewed in the direction of propagation of the electron beam, there are successively provided behind a cross-over an accelerating prefocusing lens, between the second and third grid of the electron gun, and a main focusing lens. The diameter of the aperture in the third grid (the second lens electrode) is smaller than twice the diameter of the aperture in the second grid (the first lens electrode) and the effective spacing S-eff between the second and third grid is smaller than 1 mm. S-eff is defined as the minimum of the function -.DELTA.V/E(z). Herein, .DELTA.V is the voltage difference between the third and the second grid and E(z) is the electric field strength between the third and the second grid on the axis as a function of the place z on the axis. With such an electron gun a smaller spot is obtained with less haze than with guns according to the traditional construction at comparable beam currents. This is because the spherical aberration of the main focusing lens and the spherical aberration in the electron beam in the prefocusing lens compensate each other to a certain extent, as a result of which the electron gun as a whole produces less aberration. It is necessary to use a strong prefocusing lens which is situated in the correct location with respect to the cross-over. With such a prefocusing lens the boundary rays of the electron beam are bent inwardly in such manner that in the main focusing lens they are no longer boundary rays.
A third manner to improve the spot quality is described in Netherlands patent application Ser. No. 7902868 laid open to public inspection. This improvement is obtained by using a second grid which is thick as compared with the second grid of other guns, a strong electric field between the second and the third grid, and/or an increased object distance of the main focusing lens.
A fourth manner of improving the spot quality is described in German patent application Ser. No. 3130137 laid open to public inspection. This improvement is obtained by providing after the cross-over a delaying prefocusing lens so that the outermost electron rays of the electron beam form a second cross-over for the main focusing lens. As a result of this the spherical aberration of the beam in the main lens is reduced and a spot is obtained having small dimensions only at higher beam currents.
In the last-mentioned three manners of improving the spot quality the location of the cross-over with respect to the prefocusing lens is very critical. It is therefore not beneficial to use the first grid according to the U.S. Pat. No. 4,358,703 with which an astigmatically-focused electron beam having two focal lines instead of one cross-over is obtained, without further measures in the electron guns according to the last-mentioned three patent applications. If one of the focal lines has the correct location relative to the prefocusing lens, the other focal line does not, spot quality improvement occurs only in one direction. Nevertheless there exists a need for an astimagtically-focused electron beam. For example, in self-converging display tube systems having a large deflection angle (for example 110.degree.) it is generally necessary, in order to avoid too much vertical haze in the corners of the display screen, to give the electron beam(s) in the deflection plane a smaller cross-section in a direction which coincides with the direction of deflection in which the deflection coils form a positive electron lens.